787. 归并排序

归并排序

#include<iostream>
using namespace std;

const int N = 100010;

int q[N], temp[N];
int n;

void merge_sort(int l, int r){
    if(l == r) return;
    
    int mid = l + r >> 1;
    
    merge_sort(l, mid), merge_sort(mid + 1, r);
    
    int i = l, j = mid + 1, k = 0;
    while(i <= mid && j <= r)
        if(q[i] < q[j]) temp[k ++] = q[i ++];
        else temp[k ++] = q[j ++];
        
    while(i <= mid) temp[k ++] = q[i ++];
    while(j <= r) temp[k ++] = q[j ++];
    
    for(int i = 0, j = l; i < k; i ++, j ++) q[j] = temp[i];
}

int main(){
    cin >> n;
    
    for(int i = 0; i < n; i ++) cin >> q[i];
    
    merge_sort(0, n - 1);
    
    for(int i = 0; i < n; i ++) cout << q[i] << ' ';
    
    return 0;
}

逆序对

#include<iostream>
using namespace std;

const int N = 100010;

#define LL long long

int q[N], temp[N];
int n;

LL merge_sort(int l, int r){
    if(l == r) return 0;
    
    int mid = l + r >> 1;
    LL ans = merge_sort(l, mid) + merge_sort(mid + 1, r);
    
    int i = l, j = mid + 1, k = 0;
    while(i <= mid && j <= r)
        if(q[i] <= q[j]) temp[k ++] = q[i ++];
        else{
            temp[k ++] = q[j ++];
            ans += mid - i + 1;
        } 
        
    while(i <= mid) temp[k ++] = q[i ++];
    while(j <= r) temp[k ++] = q[j ++];
    
    for(int i = 0, j = l; i < k; i ++, j ++) q[j] = temp[i];
    return ans;
}

int main(){
    cin >> n;
    
    for(int i = 0; i < n; i ++) cin >> q[i];
    
    cout << merge_sort(0, n - 1) << endl;
    
    return 0;
}